1. Field of the Invention
This invention relates to an improved process for brazing reinforcing sleeves within heat exchange tubes which avoids the creation of undesirable gaps in the resulting braze joint.
2. Description of the Prior Art
Processes for brazing sleeves within the heat exchange tubes of nuclear steam generators are known in the prior art. In such processes, a sleeve which is circumscribed by a ring of brazing alloy at one or both ends in slid across a section of the tube whose walls are in need of repair due to corrosion or denting. The end of the tube having the ring of brazing alloy is then typically hydraulically expanded so that it snugly engages the inner walls of the tube. In order to create a water-tight seal between the sleeve and the interior walls of the tube, a brazing heat is applied across the expanded end of the tube. The brazing alloy melts, and forms a water-tight braze joint between the sleeve and the interior walls of the tube. Such sleeving operations are often performed across the longitudinal sections of the heat exchange tubes which extend through the openings of the tubesheet and support plates of the nuclear steam generator due to the tendency of the walls of these tubes to corrode and dent in these regions.
While the aforementioned one-step brazing process frequently results in water-tight braze joints, problems may arise when sludge deposits become tightly wedged in the annular space between the walls of these tubes and the bores or other openings in the tubesheets and support plates through which these tubes extend. Under such circumstances, the sludge deposits may bind the outer walls of the tubes so tightly against the walls of the bores in the tubesheets and support plates that the tubes are prevented from expanding longitudinally when the brazing heat is applied to the tube and sleeve. Consequently, these tubes will expand radially instead of longitudinally in the area where the brazing heat is applied. Such a radial expansion in the area of the braze joint creates undesirable gaps in the joint which may seriously jeopardize the integrity of the water-tight seal the braze joint is intended to produce. Of course the extent to which such undesirable gapping occurs varies widely between any group of sleeve tubes due to the varying amount of binding forces the sludge deposits surrounding these tubes apply when a brazing heat causes them to expand. However, in those tubes where the sludge deposits apply a substantial amount of resistance to longitudinal tube expansions, the amount of gapping which occurs will, at the very least, significantly jeopardize the ability of the braze joint to create the intended water-tight seal.
In order to overcome the gapping problem associated with such one-step brazing processes, a two-step brazing process was developed. This process is fully described and claimed in U.S. patent application Ser. No. 634,336, filed July 25, 1984 by John Driggers and assigned to the Westinghouse Electric Corporation. In this process, heat is applied across a longitudinal section of the tube which does not include the brazing region of the sleeve in order to thermally create a radial expansion in this first section of the tube. Next, a brazing heat is applied across a second section of the tube which surrounds the brazing region of the sleeve in order to braze the sleeve into the tube. The tensile stresses generated in the tube by the first heating step tend to balance out any radial expansion that the tube undergoes at the site of the brazed joint, and greatly reduces gaps between the sleeve and the tube in the braze joint.
While this two-step brazing process represents a substantial advancement in the art, it unfortunately does not solve all of the gapping problems which may occur between sleeves and tubes in the sludge-locked tubes which are often present within nuclear steam generators. Despite the fact that a stress-absorbing bulge or radial expansion is induced in the tube near the vicinity of the brazing region between the tube and the sleeve, a smaller, but still significant gap-producing bulge can occur around the vicinity of the ring of brazing alloy as a result of the application of a brazing heat thereto. The resulting gap can cause the liquified brazing alloy to flow too far along the longitudinal axis of the tube in the small annular space between the tube and the sleeve, thereby thinning out the brazing material to an extent which sometimes results in a non-watertight braze joint. The problem is particularly acute where it is desired to braze a reinforcing sleeve across a section of tubing which has been corroded in the region around a support plate in the steam generator, because the lower ends of such reinforcing sleeve must be brazed to complete the sleeving operation. In such lower braze joints, an upward flow of brazing material (caused by capillary forces) is highly desirable. However, even a small gap between the sleeve and the tube can weaken the capillary forces to such an extent that gravity causes the melted brazing material to flow downwardly, away from the desired brazing region. While the small gap produced around the lower brazing region might be reduced by inducing a second radial expansion at another location close to, but not including the ring of brazing alloy, the resulting process would consume an inordinate amount of time, and would not always produce the desired braze joint.
Accordingly, there is a need for a brazing process which is capable of reliably providing a water-tight braze joint between a sleeve and a tube in a heat exchanger with a minimum amount of time. Moreover, it would be desirable if this process could be easily implemented with existing brazing heaters.